Particulate materials, powders and bulk solids (collectively referred to hereinafter as “comminuted materials” or “bulk materials”) are widely used in the processing of a variety of particles, powders and particulate solids, including but not limited to those used in foods and food additives, pharmaceuticals, chemicals, minerals, detergents, paints, cement, powdered metals, plastics and cosmetics (the terms “particles”, “powders”, “particulate solids”, and “particulate materials” are used interchangeably herein). In a production operation involving particulate materials, successful handling, storage and flow of such materials is an important part of overall plant design. Assurance of an even flow of particulate materials between the process stages is sought, with minimum alteration in the quality of the stored product. Since particulate solids that are stored and/or move in mass quantities depend on both individual particles and inter-particle interactions, it is desirable for plant designs to provide reliable flow of powders and other bulk materials.
Throughout the process industries, hoppers and receptacles are used extensively for the storage and transfer of bulk solids. Flowability of a material, however, may change depending on the condition of the stored material and type of hopper structure (including the hopper's material composition). The flow can be funnel flow (defined as flow from a hopper in which all particulate movement occurs through a central core above the hopper outfeed aperture with no movement occurring along all or part of the hopper wall, as illustrated in FIG. 1) or mass flow (defined as flow from a hopper in a manner such that all of the material is in motion and movement occurs along all of the hopper wall, as illustrated in FIG. 2). As illustrated in FIG. 1, funnel flow is realized when a hopper 10 has an interior wall surface 12 that is too shallow or too rough for the smooth passage of bulk material thereagainst. Consequently, material falls preferentially through a funnel-shaped channel 14 located directly above a hopper outfeed aperture 16 while material outside of this flow is stagnant, resulting in first-in last-out flow behavior.
Common drawbacks associated with funnel flow in hoppers include rat holing, segregation, stagnation, avalanching and arching in the proximity of the outfeed aperture. This behavior leads to particle spoilage, product deterioration, safety hazards, sanitary issues and equipment damage. A variety of solutions have been proposed to overcome defects inherent in funnel flow behavior, including hammering or vibration of the hopper and material stored therein or alternatively by using flow corrective inserts. Such corrective methods, however, are ineffective for cohesive materials that are sensitive to over-compaction. In addition, impellers disposed within a hopper can be employed to scrape interior hopper walls for extraneous particulate material. Such impellers, however, require extensive design, construction and maintenance in order to ensure uninterrupted particle flow.
It is therefore desirable to provide a feeder that effects mass flow of particulate materials in a predictable and controllable manner, so as to provide controllable flow rates. Such a feeder configuration achieves first-in first-out flow behavior for a wide array of particles.